Premature birth is the number one perinatal health problem in the USA. Relatively little attention has been paid to the function of milk-borne hormones and their potential implications in neonatal health care. Transfer of maternal hormones to the fetus in utero by the placenta ceases with birth; thereafter, milk is the only means of humoral signal transfer from the mother to the newborn. Milk contains hormones and other biologically active substances that may aid the adaptation of the newborn to extrauterine life. This role of milk and the mammary gland may be even more important in prematurely born infants, who normally would still be under placental influences. Based on similarities in their nutritive, immunological, and endocrine functions, the hypothesis has been developed that the placenta and the mammary gland are functionally analogous organs. As an initial objective toward testing this hypothesis, a prolactin(PRL-) secreting cell line has been established from a novel animal model: the short-tailed gray opossum (Monodelphis domestica) as a cost-effective source of species-specific hormone. M. domestica is a marsupial species; thus, it represents a branch of the phylogenetic tree that diverged from the eutherian (placental) mammals 100 million years ago. Marsupials do not have a chorioallantoic placenta and their pups are born at a very early stage of fetal development. The "extrauterine fetus" is readily accessible for experimentation and (by virtue of parallelism between ontogenetic and phylogenetic development) may serve as a model for the milk-related maternal modulation of the developing endocrine system of the premature human infant. The specific aims of the proposal are: 1. To purify opPRL from culture medium conditioned with immortalized opossum pituitary cells (chromatographic fractions will be monitored for PRL-like bioactivity in Nb2 lymphoma assay); 2. To develop a species-specific antibody in rabbits and a homologous radioimmunoassay (RIA) for opPRL; 3. To evaluate immunoreactive opPRL in maternal plasma and milk during lactation and in neonatal plasma during ontogeny, and to investigate the source of milk- borne opPRL by demonstrating transfer of radiolabeled PRL into milk in vivo, and PRL synthesis and secretion by cultured opossum mammary epithelial cells in vitro; and 4. To demonstrate the transfer of metabolically labeled, immunoprecipitable opPRL via milk into the circulation of the offspring during ontogeny. The proposed studies will lead to the development of the first species-specific PRL antiserum in M. domestica, thus providing a valuable experimental tool for pituitary, PRL and mammary gland research in the only laboratory-adapted marsupial species. The experiments will elucidate the sources of milk-borne PRL and will verify if milk is a significant exogenous source of PRL for the neonate/extrauterine fetus.